Lecture No. 21 & 22

Subject: Hot Weather Concreting

Objectives of Lecture:

  • To explain the problems and precautions associated with hot weather concreting

Problems Encountered

in Hot Weather Concreting

Hot weather can create following difficulties in fresh concrete:

  • increased water demand
  • accelerated slump loss
  • increased rate of setting
  • increased tendency for plastic cracking
  • difficulties in controlling entrained air
  • critical need for prompt early curing

If water is added to concrete to satisfy the increased water demand on account of hot weather, the properties and serviceability of the hardened concrete will be adversely affected resulting in the following:

  • decreased strength
  • decreased durability and watertightness
  • nonuniform surface appearance
  • increased tendency for drying shrinkage

Effects of High Concrete Temperatures Due to Hot Weather

1.Loss of slump

Increase in concrete temperature causes significant loss of slump that is often unadvisedly compensated for by adding more water at the jobsite

2.Increased water requirement

  • At higher temperatures a greater amount of water is required to hold slump constant than is needed at lower temperatures

As shown in Fig. 11-1, for constant slump and maximum size of aggregate, the water content increases with increase in concrete temperature

(at normal temp. of 50˚F the water required for 3 in. slump is 270-lb/yd3 whereas at an increased temperature of 100˚F the water requirement is increased by about 33-lb/yd3)

3.Reduced strength

If the additional demand of water on account of high concrete temperature is satisfied by adding water at jobsite, the w/c ratio of concrete will be increased reducing the compressive strength of concrete as shown in Fig. 11-2.

4.Reduced setting time

  • High temperatures increase the rate of concrete hardening (i.e. reduce the setting time) and therefore shorten the length of time within which the concrete can be transported, placed, and finished
  • As can be seen in Fig. 11-3, setting time can be reduced by 2 or more hours with a 20˚F increase in concrete temperature
  • Retarding admixtures can be beneficial in offsetting the accelerating effects of high temperature

Precautionary Measures Adopted in Hot Weather Concreting

1. Cooling concrete materials

  • The usual method of cooling concrete is to lower the temperature of the concrete materials before mixing
  • In hot weather the aggregates and water should be kept as cool as practicable, as these materials have a greater influence on temperature after mixing than other components
  • The approximate temperature of the freshly mixed concrete can be calculated from the temperatures of its ingredients by using the following equation:

where

T = temperature of the freshly

mixed concrete

Ta, Tc, Tw, and Twa= temperature of aggregates, cement, added mixing water, and free water on aggregates, respectively

Wa, Wc, Ww, and Wwa= weight of aggregates, cement, added mixing water, and free water on aggregates, respectively

  • Fig. 11-4 shows graphically the effect of temperature of materials on the temperature of fresh concrete
  • Out of the materials in concrete, water is the easiest to cool. Therefore, mixing water from a cool source should be used to significantly lower the temperature
  • Water can be cooled by refrigeration, liquid nitrogen, or ice
  • Fig. 11-5 shows a liquid nitrogen water cooling installation at a ready mixed concrete plant
  • Ice can be used as part of the mixing water provided it is completely melted by the time mixing is completed
  • When ice is added as part of the mixing water, the effect of heat of fusion of the ice must be considered, so the equation for temperature of fresh concrete is modified as follows:

where Wi is the weight in pounds of ice

  • Crushed or flaked ice is more effective than chilled water in reducing concrete temperature. Fig. 11-6 shows crushed ice being charged into a truck mixer prior to the addition of other materials
  • Since aggregates represent 70 % to 85 % of the total weight of concrete they have a pronounced effect on the fresh concrete temperature

To lower the concrete temperature by 10˚F, only a 15˚F reduction in the temperature of the aggregates are required

  • There are several simple methods of keeping aggregates cool
  • aggregates stockpiles should be shaded from the sun and kept moist by water sprinkling
  • aggregates can be immersed in cold-water tanks, or cooled air can be circulated through storage bins
  • vacuum cooling can reduce aggregate temperatures to as low as 34˚F

2.Preparation before concreting

Before concrete is placed, following precautions should be taken during hot weather to maintain or reduce concrete temperature:

  • Mixers, chutes, belts, hoppers, pump lines, and other equipment for handling concrete should be shaded, painted white, or covered with wet burlap to reduce solar heat
  • Forms, reinforcing steel, and subgrade should be fogged or sprinkled with cool water just before the concrete is placed
  • For slabs on ground, it is a good practice to moisten the subgrade the evening before concreting
  • Restricting concrete placement to early morning, evening, or nighttime hours during extremely hot periods, especially in arid climates

This practice has resulted in less thermal shrinkage and cracking of thick slabs and pavements

3.Transporting, placing, finishing

  • Transporting and placing of concrete should be done as quickly as practical during hot weather because delays contribute to loss of slump and an increase in concrete temperature

For this purpose, sufficient labor and equipment must be available at the jobsite to handle and place concrete immediately upon delivery

  • Prolonged mixing, even at agitating speed, should be avoided and the discharge time of concrete from ready mixed concrete drum should be limited to 1 hour or even 45 minutes
  • Since rapid drying of the concrete at the surface may cause plastic shrinkage cracking, floating should be done promptly after the water sheen disappears from the surface or when the concrete can support the weight of a finisher

4.Measures against plastic shrinkage cracking

  • Plastic shrinkage cracks are cracks that sometimes occur in the surface of freshly mixed concrete soon after it has been placed and while it has been finished (Fig. 11-7)

Plastic shrinkage cracks appear mostly on horizontal surfaces and can be subsequently eliminated if preventive measures are taken

  • Plastic shrinkage cracks occur when water evaporates from the surface faster than it can appear at the surface during the bleeding process
  • The crack length is generally from a few inches to a few feet in length and they are usually spaced in an irregular pattern from a few inches to 2 ft apart
  • The following conditions, singly or collectively, increase evaporation of surface moisture and therefore increase the possibility of plastic shrinkage cracking:
  • high air and concrete temperature
  • low humidity
  • high wind velocity
  • For a given set of values of temperature, humidity, and wind velocity the rate of evaporation (in lb/ft2/hour) could be determined using the chart as shown in Fig. 11-8.
  • There is no way to predict with certainty when plastic shrinkage cracking will occur
  • Cracking is possible if the rate of evaporation exceeds 0.1 lb/ft2/hour, and precautionary measures are almost mandatory when the rate of evaporation exceeds 0.2 lb/ft2/hour
  • Following precautionary measures should be adopted to minimize the possibility of plastic shrinkage cracking:
  • moisten the subgrade and forms
  • moisten concrete aggregates that are dry and absorptive
  • erect temporary windbreaks to reduce wind velocity over the concrete surface
  • erect temporary sunshades to reduce concrete surface temperatures
  • keep the freshly mixed concrete temperature low by cooling the aggregates and mixing water
  • protect the concrete with temporary coverings, such as polyethylene sheeting, during any appreciable delay between placing and finishing
  • reduce time between placing and start of curing by eliminating delays during construction
  • protect the concrete immediately after final finishing to minimize evaporation

5.Curing and protection

Curing and protection are more critical in hot and cold weather than in normal weather

  • For curing in hot weather the retaining forms should be loosened as soon as practical without damage to the concrete. Water should then be applied at the top exposed concrete surfaces and allowed to run down inside the forms
  • Curing water should not be excessively cooler than the concrete. Otherwise, due to temperature differentials between the concrete and curing water thermal stresses will be induced causing concrete cracking
  • The need for moist curing is greater during the first few hours after finishing
  • To prevent the drying of exposed concrete surfaces, moist curing should commence as soon as the surfaces are finished and continue for at least 24 hours
  • Application of a curing compound for protecting the cured surfaces from drying during hot weather should be preceded by 24 hours of moist curing

6.Use of retarding admixture

  • In hot weather a retarding admixture may be beneficial in delaying the setting time
  • Retarding admixtures should conform to the requirements of ASTM C 494 Type B

7.Reducing heat of hydration

  • Precautionary measures must be taken to cope with the generation of heat and attendant thermal volume changes to control cracking

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